Not just carbon, my aluminum bike (and many others I've seen) also features a downward sloping top tube.
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KibbeeSep 7 '13 at 1:01

Just a hunch here, but a sloped top tube allows less material to be used. Since the seat tube is angled, the shortest distance between the headtube and the seat tube is found by placing the top tube at an downward angle. This also allows for a shorter seat tube, using even less material. I'm guessing this is less strong, but that carbon bikes are able to withstand the forces, because of the larger tubes used, whereas steel tubes cannot handle this, and hence, have to be built the "old" way.
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KibbeeSep 7 '13 at 1:19

@Kibbee - But then they need to use a longer seatpost, which is relatively heavy and expensive. I kinda suspect that they're raising the front as much as dropping the back, and it lets them pit a hair more "beaf" in the the headset area, where the strength is needed. Plus they can and it looks sexy.
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Daniel R HicksSep 7 '13 at 1:36

Yeah, it's just a hunch, but the minimum top tube length on the old steel bike would occur around the top of the decal. You could also just have a sloping top tube, without changing the seat tube, leaving it looking somewhat like old-style women's bike, although with a slightly less angled top tube.
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KibbeeSep 7 '13 at 1:53

There's a couple reasons I can think of for this off the top of my head. Most steel bikes you see are going to b older than the aluminium and carbon ones you see, with the more modern frame shaping, and I believe the longer seat post is designed to soften some of the harshness in these modern materials.
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alexSep 7 '13 at 15:07

2 Answers
2

The only real advantage to a compact geometry road bike is for the manufacturers. Since standover- which is the distance from the top tube to your crotch when you straddle the bike with your feet on the ground- is increased, one size of frame will fit a wider height range of people and therefore the manufacturer does not have to make as many different frame sizes. This is valuable for aluminum and steel bike frames, but pays off the most in the world of monocoque carbon frames as each size requires its own mold(s) and for higher end bikes, its own layup schedule. Final fitting is done with saddle height & fore/aft position along with steerer stack height, stem length & angle. Ultimately there is no difference in fit between compact and classic road bike geometry.

Compact geometry frames use ever so slightly less material in the frame, but primarily they are just the new aesthetic of road bikes as compared to those of the 1980s era and before, and that era of bike was typically steel.

Q: You generally see the top tube on steel bikes being parallel to the ground, whereas on a lot of carbon bikes it's angled. Why is that?

A: It is because most steel bikes you see are old, designed and built in an age before the sloping top-tube was conceived (for Giant bicycles by Mick Burrows in the mid-90's I believe). So aside from recent artisan-built steel, most steel bikes will have horizontal top-tubes (parallel to the ground). By contrast, most carbon bikes were made as jm2 above has stated, with saving money on mould-making a key consideration, hence the slope. Interestingly, many custom-made steel bikes are now made sloping as well, because because its seen as a more modern look. Yet perversely some of the poshest new carbon frames (eg some Legend, Parlee and BMC frames) are now made with horizontal, or near-horizontal top tubes, because their customers want a more 'traditional' look. What goes around comes around.

Oh, Squoval fom Cervelo? Asymmetric forks from Pinarello? This stuff was never truly new; just re-invented. 20th century Britain took such a dim view of professional cyclists that amateur riders were banned even from having the name of a frame manufacturer on the tubing, hence builders like Hetchins came up with curly chainstays, EG Bates with s-shaped forks and Granby with square-tubed chainstays - all in a bid to be distinctive.
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VaVaVeteranSep 8 '13 at 19:49